A relief map of the Moon’s South Pole-Aitken basin, based on JAXA data. The purple and blue areas represent low relief. The black ring is an old estimate of its extent; the magenta and pink rings represent its inner and outer rims, respectively.JAXA/Ittiz/Wikimedia Commons; CC BY-SA 3.0

As I’m sure you’ve spotted by now, a Chinese lander by the name of Chang’e-4 (named rather appropriately after a legendary lunar deity) made history: For the very first time, a lander has made a soft landing on the Moon’s far side. Specifically, it landed in the South Pole-Aitken basin, which is a depression feature roughly 2,500 kilometres (1,550 miles) across, and around 8 kilometres (5 miles) deep at its core.

Those of you that follow me know that I’m enthralled by the Moon; it’s geological curiosities and its electrical envelope are simply magic to me. Plenty has been written about the landing of Chang’e-4 itself, and about the differences between the near and far sides of the Moon – including by yours truly – but not that much attention has yet been paid to the bizarre nature of the landing site itself.

So, let’s dive in, shall we?

Chang’e-4 landed in the Von Kármán crater, generated by a meteorite impact long ago. This crater, however, is in the far more colossal South Pole-Aitken (SPA) basin, by far the largest impact crater on the Moon. It makes up nearly a quarter of its entire surface area.

The size of an impact crater depends on many things, from the geology of the surface to the angle, speed and type of impactor. One 2012 study suggested a 170-kilometre (105-mile) entity slammed into the South Pole of the Moon pretty much head on moving at 10 kilometres (6.2 miles) per second. Another published that year reckoned that a 200-kilometre (124-mile) object moving at 15 kilometres (9.3 miles) per second hit the South Pole at an oblique, 45-degree angle.

Either way, it was huge: about the same length as the distance between Washington DC and Philadelphia. It may have been powerful enough to knock the Moon over, as one 2007 piece in New Scientist reported. It’s possible that the original impact took place near the equator – which would be exposed to the paths of most objects in the Solar System – and that the impact destabilized the Moon’s orbit, causing it to tip over and move the basin to the South Pole.

As the dual nature of the SPA basin’s name suggests, the South Pole and the Aitken Basin aren’t synonymous. The basin isn’t centred exactly on the South Pole, but it overlaps it. This is perhaps a feature of the numerous other impactors slamming into the Moon causing further wobbles and destabilizations over time.

As noted by NASA, it’s clearly the oldest impact crater on the Moon. They know this because the SPA is a basin riddled with craters. It’s quite literally covered in them, which means that plenty of impacts took place after the older, larger crater formed. Using knowledge of how often that impactors careen into the Moon, researchers are 100 percent confident that it’s pretty darn ancient – perhaps around 4.26 billion years old, not too long after the Moon as a whole formed.

Exactly how old, though, is a matter of debate. No direct samples – as of yet – have been taken from SPA in order to date it using radiogenic analyses, where the decay of radioactive materials act as a sort of chemical clock. It is, however, at least 3.9 billion years old, and the clue’s right there in the name.

SPA is a basin, and the Moon’s basins all formed roughly at the same time: 3.9 billion years ago, according to the lunar samples already taken from those areas on the near side. Just before that time, closer to the Moon’s fiery birth, the lunar surface was likely a single crustal shell overlying a bubbling ocean of magma. Then came the Late Heavy Bombardment (LHB), taking place 4.1 to 3.8 billion years ago. During the LHB, the Solar System’s rogue rocky bodies flew around in huge quantities and with reckless abandon, causing plenty more impact events on larger rocky bodies than we see taking place today.

This gave rise to the dark mare, the lunar “seas” that dominate the near side of the Moon that, to some, look like a person’s face. These basins are mindbogglingly enormous floods of lava that poured out of cracks in the Moon’s crust when asteroids pierced through it.

Those of you with a keen eye might notice that the far side of the Moon is home to very few basins. Instead, it’s dominated by terrae, which are volcanic protuberances of an entirely different form. So why the difference?

The far side of the Moon.NASA Goddard

Right from when the Moon formed (via a giant impact event early on in the Earth-Moon story), Earth’s far stronger gravitational field acted as a break on the puny Moon and its rotation on its own axis. That meant it became “tidally locked”, in that it rotated in such a way that we only ever see one face of it from our pale blue dot of a planet – what we’ve come to call the near side.

The Moon was once far closer to Earth, and both were once molten. This meant the near side of the Moon was continuously cooked and heated by Earth’s fiery glow. Consequently, this prevented the near side’s crust from cooling and thickening properly, and vaporized minerals were ferried over to the far side.

Ultimately, this led to a thinner near side crust, something asteroids could more easily pierce and produce those lava-filled basins. This may also explain why volcanism on the near side finished 1.2 billion years ago, while the more resilient, thicker far side lacks any volcanism from 2.5 billion years ago onwards.

This makes the SPA basin something of an anomaly. It happens to be on the far side of the Moon, which implies the space cannonball that excavated it had somewhat thicker crust to smash through than any smashing into the near side. That, along with the crater’s size, strongly indicates that it was one of the most powerful impact events in the Solar System’s history.

Unsurprisingly, thanks to this huge impact, there’s also a fair bit of ancient volcanism observed in the SPA basin too. According to this 2018 study on the region, there are at least 129 mare lava flows preserved in and around the basin, which are anywhere from 2.2 to 3.7 billion years old – the oldest of which can be found in the Jules Verne crater.

The Moon underwent two periods of major volcanism: one 3.6-3.2 billion years ago, and one 2.5-2.2 billion years ago, on both its near and far sides, albeit it with vastly different volumes and somewhat differing styles. Weirdly, though, compared to the near side basins, there doesn’t appear to be a second peak in volcanic activity, that 2.5-2.2 billion years ago phase, in the SPA basin.

So what’s with the missing volcanism? Well, that 2018 paper reckons that the impact that created it was so ginormous that it stripped away much of the crust in the region. That meant the underlying magmatic sources were more exposed to the frigid realm of space, allowing it to quickly cool and prevent much eruptive activity from taking place thereafter. Incidentally, it also might lack the radioactive elements found elsewhere in the Moon that could keep rock molten over long time periods.

Despite the impactor managing to penetrate through perhaps the entire thickness of the Moon’s crust, then, just 3-4 percent of this huge basin is filled with basaltic lava deposits – a far cry from the prolific outpourings seen on the Moon’s near side.

Plenty of uncertainties and mysteries remain – particularly over the formation of the basin and its chaotic chemical makeup. After all, there’s only so much geoscience you can do from Earth. Fortunately, spacecraft like Chang’e-4 – a lander-rover combo equipped with, among other things, ground-penetrating radar – may be able to shed some light on. It, along with future missions to the site, will help to throw some fire at these scientific shadows, and, over time, the Moon’s secrets will continue to be revealed.